The fabrication of highly integrated electrical circuits with small structural dimensions requires special structuring procedures. One of the most common procedures is the so-called lithographic structuring method. This method comprises applying a thin layer of a radiation-sensitive photoresist on the surface of a semiconductor substrate disc, also referred to as wafer, and exposing the same by radiation transmitted through a lithographic mask. In the so-called photolithography, electromagnetic radiation is used. During the exposing step, a lithographic structure located on the mask is imaged on the photoresist layer by means of an exposure tool including the mask. Thereafter, the target structure is transferred into the photoresist layer and subsequently into the surface of the wafer by performing development and etch processes.
One main demand of the semiconductor industry is the continuous power enhancement provided by increasingly faster integrated circuits which is interrelated to a miniaturization of the electronic structures. Thereby, the attainable resolution of the structures is generally limited by the wavelength, of the applied radiation. In the course of this development, lithographic methods are performed with radiation having ever smaller wavelengths. At present, the smallest exposure wavelength used in the semiconductor production is 193 nm which enables the fabrication of minimal feature sizes of approximately 70 nm. For the nearer future, the application of the so-called 193 nm immersion lithography is intended, thus allowing minimal feature sizes of about 50 nm.
In order to achieve still smaller feature sizes, the so-called extreme ultraviolet lithography (EUVL) is being developed which is based on the application of electromagnetic radiation in the extreme ultraviolet (EUV) region with a wavelength of 13.4 nm. According to plans of the semiconductor industry, the EUV-lithography is considered to be used for the fabrication of dense sub-40 nm and isolated sub-25 nm structures by the end of this decade.
Since no refractive materials (lenses) exist for EUV radiation, the radiation has to be reflected by special mirrors, i.e. multilayer reflection elements, which are used in the corresponding exposure systems for the exposure tools and for the lithographic masks. A typical EUVL reflection mask comprises a carrier substrate, a multilayer reflection layer provided on the carrier substrate, and a patterned absorber stack provided on the multilayer reflection layer which defines the lithographic structure. The reflection layer usually consists of a number of Si/Mo-bilayers, which are disposed upon each other. The patterned absorber stack typically comprises a buffer layer consisting for example of SiO2 and an absorber layer consisting for example of Cr or TaN. The buffer layer of the absorber stack serves for protection of the multilayer reflection layer during the fabrication of the reflection mask, in particular regarding repair processes of pattern defects.
In order to provide a high contrast of the so-called aerial image, i.e. the intensity distribution of the radiation imaged on a wafer, the absorber stack of a conventional EUVL reflection mask in general has a thickness which is relatively large compared to the wavelength of the radiation, and thus comprises a geometry with a high aspect ratio, i.e. a high ratio between the thickness and a lateral dimension. A high aspect ratio geometry of an absorber stack provokes some disadvantages, however. Since a lithographic reflection mask is irradiated in an oblique angle of approximately 6° relative to a vertical plane, shadowing effects generally occur. Due to an absorber stack with a high aspect ratio, these shadowing effects strongly affect the imaging quality of a lithographic process. In particular, structure displacements and alterations of lateral dimensions of the structures, also referred to as critical dimension (CD), occur.
Another consequence of a high aspect ratio geometry of an absorber stack is a reduction of the lithography process window, i.e. the range of possible defocus and intensity dose values corresponding to a tolerable range of target critical dimension values. In general, focus and intensity dose settings of an exposure tool are not constant in a lithographic process but comprise variations, e.g. due to a wafer's non-flatness or due to fluctuations caused by the exposure tool. Due to such variations, a target critical dimension lies within a defined rage. A reduction of the process window increases the danger of rejections of processed wafers having structures with intolerable critical dimension values.
In order to solve these problems, different solution concepts have been proposed. One concept is based on providing reflection masks having structured multilayer reflection layers instead of patterned absorber stacks in order to avoid shadowing effects, as e.g. described in DE 101 23 768 A1. However, the application of such reflection masks having patterned reflection layers requires the additional development of defect inspection and repair methods. But in the case of dark defects, i.e. an area where too many reflective multilayers have been removed, repairing is practically impossible. Moreover, structured multilayer reflection layers have a lower stability in particular concerning cleaning procedures.
Other concepts include methods for determining improved absorber stack layouts by performing optical or aerial image simulations in which structure elements of absorber stacks are laterally displaced in an iterative manner in order to compensate for or minimize the displacements of the target structures due to shadowing effects. Such methods, however, which are alike to optical proximity correction techniques (OPC) performed on standard transmittance masks, are very complex and time-consuming.